A thermal type flow sensor according to a prior art is disclosed in Japanese Patent Application Publications No. 2002-48616 and No. H11-258021. FIG. 63 shows a sensor 5000 according to the prior art. This sensor 5000 detects flow of fluid with measuring heat absorbed by the fluid.
Specifically, the sensor 5000 includes a heater for generating heat, and the heat is absorbed in the fluid. In this case, as amount of the flow of the fluid becomes large, the absorbed heat increases. Thus, the sensor 5000 detects the flow on the basis of the absorbed heat. For example, the heater is energized so as to maintain the temperature nearby the heater at a predetermined temperature. In this case, the heater consumes electric power, which corresponds to the absorbed heat, so that the electric power consumption in proportion to the flow of fluid is measured.
Alternatively, a thermal type flow sensor having a heater and a detector controls the heater so as to maintain a detected temperature at a predetermined temperature. The detector detects the temperature nearby the heater, the temperature including information of heat absorbed in fluid. Thus, the sensor detects flow of the fluid in proportion to the absorbed heat. In general, the heater works as a detector for detecting its own temperature.
The sensor 5000 includes a semiconductor substrate 5001 having a concavity. In the concavity, a sensing portion 5010 is disposed. The sensing portion 5010 detects flow of fluid. The sensing portion 5010 has a heater 5011, a pair of upstream temperature detectors 5012a, 5012b and a pair of downstream temperature detectors 5013a, 5013b. The pair of upstream temperature detectors 5012a, 5012b detects the temperature of the fluid disposed upstream from the heater 5011. The pair of downstream temperature detectors 5013a, 5013b detects the temperature of the fluid disposed downstream from the heater 5011. Further, the sensor 5000 includes an environmental temperature detector 5020 disposed on the substrate 5001. The environmental temperature detector 5020 detects the temperature of environment of the sensor 5000.
Each of the heater 5011, the pair of upstream temperature detectors 5012a, 5012b, the pair of downstream temperature detectors 5013a, 5013b and the environmental temperature detector 5020 connects to a pad 5040a–5040h through a lead wire 5030a–5030l, respectively. Then, they connect to an electronic circuit (not shown) for outputting a sensor signal corresponding to the flow.
Each of the heater 5011, the pair of upstream temperature detectors 5012a, 5012b, the pair of downstream temperature detectors 5013a, 5013b and the environmental temperature detector 5020 detects temperature by measuring its own resistance change. The electric circuit controls electric power supplying to the heater 5011 in such a manner that the temperature of the heater 5011, which is detected by its own resistance change, is set to be higher with a predetermined temperature than the environmental temperature detected by the environmental temperature detector 5020. Further, the electric circuit detects the heat absorbed by the fluid passing through the heater 5011 in accordance with the temperatures detected by the upstream and downstream temperature detectors 5012a–5013b. 
In this sensor 5000, it is required to enlarge a width W of the heater 5011 in a flowing direction Z of the fluid so that sensitivity of the temperature of the heater 5011 is improved. However, when the width W of the heater 5011 is wide, sensitivity of the flow of the fluid is reduced. Further, the energy consumption of the sensor increases.
In addition, to improve sensor sensitivity of a flow sensor and to reduce electric power consumption of the flow sensor, it is required to decrease resistance of a lead wire of the sensor. In this view, a flow sensor having a thick lead wire according to a prior art is disclosed in Japanese Patent Application Publication No. 2002-71416. In this sensor, the thickness of the lead wire is in a range between 1 μm and 2 μm so as to reduce the resistance of the lead wire.
However, the sensor having the thick lead wire includes a large step disposed on the surface of the sensor. The large step disturbs the flow of the fluid, so that detection accuracy of the sensor is decreased. Further, a contamination in the fluid may adhere to the large step, so that heat capacity or heat conductivity of the sensor is changed. Therefore, the detection accuracy of the sensor is decreased.
Further, a flow sensor having a heater and a detector made of poly silicon film according to a prior art is disclosed in Japanese Patent Application Publications No. H11-258021 and No. 2001-12985.
When the heater and the detector are made of poly crystalline silicon film, the surface of each of the heater and the detector has a concavity and convexity since the poly crystalline silicon film has a grain boundary. Therefore, a passivation film covering the heater and the detector also has a surface with a concavity and convexity. If the top surface of the sensor, i.e., the surface of the passivation film has the concavity and convexity, a stress is concentrated at the concavity and convexity of the passivation film. This stress concentration at the concavity and convexity causes decrease of pressure resistance of the poly silicon film and decrease of maximum detection value of flow of fluid. Here, the maximum detection value is defined as a value, at which the poly silicon film is broken.
Further, in the above sensor, part of the passivation film becomes thin, the part being disposed at a corner of the heater or detector. Specifically, the corner of the heater or detector is sharpened, so that the thickness of the passivation film covering the corner of the heater or detector becomes small. Especially, when the heater or detector is formed with a dry etching method, the corner is much sharpened. This partially thinned passivation film causes decrease of pressure resistance of the poly silicon film and decrease of endurance against a collision of a large dust hit the passivation film.
The above sharpening of the corner also occurs in a sensor having a heater and detector made of single crystal silicon. Therefore, a passivation film covering the corner is thinned, so that pressure resistance of the thin film portion is reduced and endurance against a collision of a large dust hit on the passivation film is decreased.
Further, a flow sensor having a passivation film made of silicon nitride according to a prior art is disclosed in Japanese Patent Application Publications No. H11-271123 and No. 2001-194201. The passivation film reinforces a thin film portion of the sensor. However, the thickness of the passivation film is limited so that damage caused by the collision of a dust hit on the passivation film is not sufficiently reduced. Further, to reduce a large tensile stress in the silicon nitride film, a thick silicon oxide film is formed between a substrate and the silicon nitride film. The large tensile stress is mainly applied to the silicon nitride film when the silicon nitride film is formed on the substrate. When the thickness of the silicon oxide film becomes large, the Young's modulus of the thin film portion is reduced. Therefore, the thin film portion is easily deformed, so that the endurance of the thin film portion is decreased.